The technique for the excavation and preparation of tunnels or of underground excavations of any type whatsoever for roads, railways, for canals or underground waterways etc., generally makes use of peripheral consolidation techniques in such way as to create a resistant surrounding for the thrust of the earth during the internal excavation and the final realisation of the work.
Nowadays such known consolidation interventions envisage multiple perforations created around all or part of the external circumference of the tunnel, within which metal, fibreglass or other suitable material reinforcements or inserts are introduced, which are filled with a jet of concrete or other means of consolidation, such as cement mixtures, gel or resins, injected at low or high pressure. These consolidating materials mix with the material present around the periphery of the tunnel being built.
With these known systems crown or umbrella stretches of the consolidated terrain are created, which allow the carrying out in safety of a smooth stretch of excavation of the tunnel. Such a system of execution takes place for further courses of consolidation that are obviously quite short (for example 15-20 m. in length) with resistant veins off axis compared to the tunnel axis, in such way as to be able to act for later stretches of consolidation and relative successive excavation stretches.
The main inconveniences of this known system of proceeding are the following:                the need for numerous and systematic interventions in making headway, to alternate with excavation operations, with use of the personnel necessarily operating in the tunnel;        the moving in alternation of the equipment necessary for realisation of the consolidation and excavation at the start and finish of each intervention of consolidation;        the setting times for the mixtures used for the consolidation and such times are generally lost and are reduced as far as possible to the minimum with possible reduction of the resistance;        the need for consolidation work on the nucleus of the tunnel being built to sustain the excavation front;        the need for superimposition of each consolidation intervention compared to the preceding and successive to have guarantees of support for the tunnel front;        the need to perform the perforations with angles not longitudinal to the tunnel to create the space for the following treatment;        the excavation of the tunnel with variable sections to compensate with plugging and final works;        lengthy times for realisation of the work.        
In cases in which the consolidation should have to be performed from above on a surface over the tunnel (tunnels with little cover) the difficulties of consolidation, at the current state of the art, are the following:                the need for perforation in a void to reach the zone inside the tunnel to consolidate;        the presence of works (sub-services, roads, houses, etc.) existing or to build on the surface;        costs of expropriation or occupation of public or private areas for execution of the consolidation;        the numerous movements on impervious bottoms or with poor resistance of the machines useful to realize the consolidation, including injection plant, pumps, etc. and all the structural elements (metal or fibreglass inserts).        
Guided perforation systems are also known for tunnels or underground channels of limited cross-section such as sewers, water ducts, channels for electric and telephone cables etc., where said guided perforation takes place by means of equipment with rotating pipes fitted with special perforating heads, such flute shaped heads or hole bottom with asymmetric headed hammers or alternatively with mud turbines, which allow performance of piloted perforations even of considerable length, such as for instance more than 800 m depending on the terrain to cross, with direction that is constantly checked with systems of directional command, such as radio, magnetic, radar, radioactive, with reference to GPS systems or with optical and electronic references, so as to allow at any time the exact location and orientation of the head in the perforation phase.
Perforation may take place with the use of air or fluids under pressure, with destruction or coring bits, with the use of hole bottom hammers, with milling cutters, tricone bits, chisels, mud turbines etc. suitable for correction of the direction of perforation.
The material that results from the perforation discharges between the walls of the drill hole and the drill rod or between the rods and the possible casing, as commonly happens with perforation equipment for wells. The perforation rods that step by step advance within the excavation are then recovered together with the perforation head and during such recovery the possible boring of the hole and the placing of casing takes place with an internal liner in case of drainage or service pipes.
As mentioned, such guided perforation equipment as known nowadays, allows only the perforation of holes or channels of limited cross-section and does not yet permit the excavation of tunnels of considerable or large dimensions, which require consolidation before the excavation operations.